1. Field of the invention
The invention relates generally to touch-sensitive panels of the type used as computer input devices and more specifically to touch-sensitive panels in which the touch brings two surfaces into closer proximity and that in turn causes changes in the physical properties of either or both surfaces that permit detection not only of the touch but also determination of its location in the panel.
2. Description of the prior art
Touch panels that work by detecting that surfaces have been brought into close proximity will be termed in the following membrane touch panels. One class of such membrane touch panels is resistive membrane touch panels. As described for example in U.S. Pat. No. 4,965,421, Epperson, Particulate spacers for touch sensitive overlay panel applications, issued Oct. 23, 1990, one of the surfaces is rigid and the other is flexible. The flexible surface is located a small distance above the rigid surface. Both surfaces conduct electricity. When a touch causes the flexible surface to come in contact with the rigid surface, a short develops between the surfaces. The short changes the resistance across the surfaces, and the change in resistance in turn can be used to determine the location of the short and from that, the location of the touch.
It is of course imperative in membrane touch panels that the gap between the surfaces be maintained when the touch panel is not being touched. Two general techniques have been developed for maintaining the gap:
placing spacers at the edges of the panel and keeping enough tension on the flexible surface that a gap between the flexible and rigid surfaces that is substantially the width of the spacers is maintained across the entire surface; and
placing spacing structures between the flexible and rigid surfaces throughout the panel. A touch causes the flexible surface to come into proximity to the rigid surface between the spacing structures. In resistive membrane touch panels, the spacing structures are insulators.
Spacers and tension have generally been used in large touch panels, for example, those employed in touch-sensitive electronic whiteboards, while the spacing structures have generally been employed in transparent touch screens designed to be placed over computer displays. A description of a touch-sensitive electronic whiteboard may be found in U.S. Pat. No. 5,790,114, Geaghan, et al., Electronic whiteboard with multi-functional user interface, issued Aug. 4, 1998, and a description of a touch screen using spacing structures throughout the panel may be found in the Epperson patent referenced above.
Both techniques for maintaining the gap have had limitations. The spacing structures used with transparent touch screens have had to be so small as to be substantially invisible to the naked eye and have consequently typically had a size on the order of 0.00005 inches. The use of spacing structures of this size obviously requires close manufacturing tolerances and the need for such tolerances has prohibited the use of the technique for large, relatively low-cost touch panels such as those used for electronic whiteboards.
The tolerances are not so close when tension is employed, but the surface must be flat, that is, it may not have bumps that are near the height of the gap, and this requirement is made more difficult by the fact that the tension that must be applied to the flexible surface to maintain the gap tends to cause the rigid surface to bow or warp. As the size of the panel increases, getting it flat in the first place and keeping it flat under the tension being applied to the flexible surface becomes more and more difficult.
Moreover, the fact that tension has been used to maintain the gap has meant that the writing surface of the touch panel has had a xe2x80x9cspringyxe2x80x9d feel. A user of the touch panel must press down hard enough with the writing instrument to overcome the springiness. Where the writing instrument makes marks of its own, as is the case with the markers used with electronic whiteboards, it is even possible that a user who does not press down hard enough may make marks on the whiteboard that are not recorded by the computer connected to the whiteboard. What""s worse, the amount of pressure required to overcome the springiness varies across the whiteboard, with the amount of pressure increasing as the writer approaches the spacers at the edges of the whiteboard. The springiness and the associated possibility of loosing information are of course particularly undesirable in an electronic whiteboard, since the whole purpose of an electronic whiteboard is to record what the users write on it. Here, too, the problem increases as the board""s size and the amount of tension required to maintain the gap increase. What""s more, the problems with springiness are only aggravated by any tendency of the board to bow or warp under the tension.
The problems caused by surface irregularities and bowing and warping can be dealt with by careful selection of materials for the surface and careful manufacturing techniques, but these add to the cost of the panels and, in the case of low-cost panels such as those required for electronic whiteboards, the problems of the tension techniques have effectively limited the maximum size of commercially practicable electronic whiteboards made using membrane touch panels. The problems caused by the springiness of the writing surface are, however, inherent in the use of tension to maintain the gap between the flexible and rigid surfaces and cannot be solved by the use of even the best materials and manufacturing techniques.
It is thus an object of the present invention to provide touch panels which do not have a springy writing surface, which may be manufactured at a low cost, and which do not have the size limitations that are presently imposed by the need for low-cost manufacture.
The invention achieves its objects by using spacing structures to separate the surfaces in large touch panels. The spacing structures make it unnecessary to use tension to maintain the gap between the flexible and the rigid surface and the spacing structures are sized and placed in a fashion which permits irregularities in the rigid surface. An elastic element permits a spacing structure to yield to pressure from a touch or a writing instrument, and thereby prevents a user who is writing on the panel from feeling the spacing structure and prevents the spacing structure from affecting the movement of the writing instrument on the panel. The invention thus makes it possible to produce large, low-cost touch panels that have a superior feel, since there is little or no springiness for the writer to overcome and the writer does not sense the spacing structures.xe2x80x9d